Drilled Shaft Testing
WSDOT uses non-destructive
testing methods to test the integrity of its drilled shaft
foundations. We
use a test called Cross Hole Sonic Logging or CSL for short.
CSL  measures
how long it takes sound waves to pass from one side of the
drilled shaft to the other.
The time that it takes for the sound waves to travel is
influenced by the material that the sound waves pass through
and the distance that the sound waves travel.
When we construct a drilled
shaft, we place tubes around the outside of the shaft.
Once the concrete is poured, we can place a sound
source in one tube and a sound receiver in the other.
We then measure how long it takes the sound to travel
from one tube to the other using a computer.
In good concrete, the sound travels straight from one
tube to the other very quickly.
If the concrete is of poorer quality, the sound is
slowed down. Likewise,
if there is some foreign material in the shaft or a hole in
the concrete, the sound has to travel a longer distance to go
around it. The longer distance causes a time delay that we can
see in the test.
We usually test half the
drilled shafts constructed, but if we find shafts that have
problems, we will test every one to make sure that the shafts
that support our structures are good.
On occasion, we encounter a
shaft that has a serious problem.
When our testing indicates there may be a problem, we
will bring a drilling machine out to the site and drill a hole
in the shaft to obtain samples of the concrete.
 The
photograph to the right shows a core sample of concrete we
recovered from a drilled shaft where the CSL test indicated
there was a potential problem.
The cement was washed from the concrete during the
placement of the concrete leaving the aggregate in the shaft.
Our Contractor injected grout under high pressure to
repair the area of poor concrete.
CSL tests are coordinated
through the Seattle Inspection Office of the Materials
Laboratory. They
can be contacted at (206) 464-7770.
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